The disclosed embodiments of the present invention relate to a transmit power control mechanism, and more particularly, to a transmit power measurement apparatus having a programmable filter device that is set at least based on a frequency response of a transmit power detection path and a related transmit power measurement method thereof.
Mobile communication has changed the way people communicate. Mobile phone has been transformed from a luxury item into an essential part of everyday life. Many wireless communication protocols have been proposed to realize the mobile communications. For example, the mobile phone may support at least one of wireless communication protocols such as the fourth generation (4G) wireless communication protocol (e.g., Long Term. Evolution (LTE) technology or LTE Advanced (LTE-A) technology) and the third generation (3G) wireless communication protocol (e.g., Wideband Code Division Multiple Access (WCDMA) technology or Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) technology).
Generally, after a digital-to-analog conversion of raw data to be transmitted by a transmitter of a wireless communication device (e.g., mobile phone), the analog transmit signal is coupled through analog components, such as one or more analog filters, a passive mixer, one or more drivers and a power amplifier. There may be other stages as well. Further, one or more of these analog components may have variable gain properties in order to adjust the gain of the signal at various stages of the transmit path. In other words, the transmitter includes a form of gain control used to control the strength of the radio frequency (RF) signal transmitted from the wireless communication device. The gain control is generally divided into discrete chunks, called “gain steps”.
The 3G/4G wireless communication protocols allow a maximum error of −/+0.5 dB in gain over all conditions (e.g., temperature, process, supply voltage, etc.) when a 1 dB power level change is made. This gain step accuracy should be met over the entire transmitter power control range. In addition, the tracking accuracy of the power control algorithm should be −/+0.1 dB. In general, a transmit power detection path may be coupled to the power amplifier output though a coupler, such that an RF sensing input is received and processed by the transmit power detection path to generate a transmit power detection signal. The transmit power detection path includes analog components and therefore has a gain setting. Hence, a power measurement error may result from gain asymmetry of the transmit power detection path. For example, the power measurement error may be an absolute measurement error in a narrow bandwidth transmission, a larger measurement variation in a wide bandwidth transmission, or both of the above in the presence of baseband (BB) hopping. Specifically, the gain asymmetry shape (i.e., gain versus frequency) varies with different gain settings, and moves up/down with temperature variation. FIG. 1 is a diagram illustrating the frequency response of the transmit power detection path. When the transmit power detection path has the first gain setting, the frequency response curve CV1 is not flat and asymmetrical around the carrier frequency with frequency offset equal to zero. When the transmit power detection path has the second gain setting, the frequency response curve CV2 is not flat and asymmetrical around the carrier frequency with frequency offset equal to zero. As shown in FIG. 1, the frequency response curve CV1 is different from the frequency response curve CV2.
Thus, there is a need to cancel/mitigate the power measurement error resulting from the gain asymmetry of the transmit power detection path.